Devices to measure cracks are generally for use by civil, geotechnical, and structural engineers in forensic monitoring of structures for differential changes in crack displacement due to foundation settlement, soil heaving, subsidence, subsurface soil compaction, temperature, moisture, and other geotechnical or construction-related factors. Furthermore, establishing crack profiles before and after geophysical events such as earthquakes and hurricanes is beneficial in assessing impact to structural integrity, which affects occupant safety and choice of remediation.
Structural cracks, regardless of cause, may appear in concrete, masonry, stone, steel welds, wood joints, or virtually any material subjected to tensile, compressive, or shear stresses and loads. Engineered separation joints, such as expansion joints, are intentional and allow adjacent surfaces to expand and contract, minimizing cracking elsewhere. Cracks and expansion joints in reinforced concrete, for example, can lead to moisture permeation and premature corrosion of reinforcing steel (rebar), dowels, and fasteners, thus reducing service life or load-bearing capacity of the structure. Depending on the structure, its intended use, and the contractual warranty between the contractor and building owner, cracks larger than a specified width may require intervention, such as grooming and epoxy injection. For example, tolerable crack widths in reinforced concrete are well documented in the American Concrete Institute publication ACI 224R-01 “Control of Cracking in Concrete Structures,” (2001; ISBN: 9780870310560).
Various devices have been developed and commercially deployed to measure crack displacement in structures, such as building foundations, bridges, dams, highways, pipelines, ships, and other high-value assets. Examples include but are not limited to rulers, caliper crack marks, thermoplastic 2D crack monitors, embedded crack sensors, and more complex electronic strain-gauge sensors with wireless connectivity. Each method offers varied benefits and tradeoffs of lifecycle cost, accuracy, weather-resistance, installation complexity, vandal resistance, measurement flexibility, and engineering traceability.
However, prior-art mechanical two-dimensional crack monitors fail to measure or account for the effects of Z-axis offsets across or perpendicular to the crack plane, either at the time of installation or throughout the crack-measurement study. Rather, they assume that adjacent surfaces are coplanar and that top and bottom plates will remain essentially flush and parallel to each other throughout the measurement service life. Although some prior instruments may account for minor Z-axis fluctuation, they do not measure or record Z-axis motion. Other devices that have been developed to measure Z-axis motion have a limited range of measurement and may rely on difficult-to-achieve and maintain manufacturing tolerances.
Mounting a crack monitor across a span that also moves along the Z-axis (perpendicular to the crack plane), however, may readily deform the measuring device, including up to complete breakage. Z-axis movement perpendicular to the crack plane is common in unreinforced masonry, or impaired reinforced concrete. Sudden seismic events or sinkholes can also impart near-instantaneous shifts in crack planes, which quickly damage instrumentation. Crack monitors are typically injection-molded from common thermoplastic such as acrylic or polycarbonate. These molded products contain a fixed, integral step, which exactly matches the plate thickness. For example, if the plate thickness is 3 mm, the fixed mounting offset is also 3 mm, such that when mated together and mounted on a coplanar surface, the top and bottom plates may slide freely and independently. However, these devices are subject to deflection or cracking when one side of the crack plane experiences a step-wise shift along the Z-axis, such that a portion of the bottom, cantilevered plate bends upward across the crack plane. Depending on the direction of Z-axis shift, the bottom plate may press up against the top overlapping plate, distorting both in an upward trajectory. Conversely, the top plate may begin to press down on the bottom plate, distorting both in a downward trajectory. Regardless of shift direction, such bending distortion often results in the measuring device resting cockeyed against the crack plane, making it difficult to obtain accurate readings due to parallax and optical shifting between the two plates. In this situation, the adhesive or fasteners used to fixedly secure the device may fail, or the device may deform to ultimate breakage. Once this occurs, all measurements of relative crack movement are permanently lost along all dimensions, and the measurement process must start anew using a replacement device.
Existing devices are often injection molded with a fixed, integrated mounting flange, and are designed to operate under the ideal assumption that the crack plane is coplanar along the Z-axis. While injection molding offers the benefit of low part cost in high volume, the higher initial capital cost of tooling precludes rapid evolution and customization of the crack measurement device to meet varying customer requirements. For example, measurement of expansion joints of bridges may require measurements spanning 5 to 10 inches or more, far greater than the typical span of economy injection-molded crack monitors. The investment in tooling for each situation becomes cost-prohibitive in all but the highest volume applications, thus limiting flexibility to adapt the measurement device to meet new or evolving customer requirements.
Furthermore, it is desirable and convenient to concurrently measure independent X, Y and Z vectors, as well as absolute magnitude, defined as the square root of (X-squared+Y-squared) in the case of a 2D measurement, or the square root of (X-squared+Y-squared+Z-squared) in the case of a 3D measurement, with or without a calipers.
In addition, many existing crack monitors print and establish the zero-offset reference point in the middle of an X-Y Cartesian grid that typically spans+/−20 mm×+/−10 mm to +/−25 mm×+/−10 mm or thereabout. In some situations, this choice unnecessarily limits the X-axis measurement range of the crack monitor by as much as a factor of two. For example, if the initial crack width is small—such as a few millimeters, which is typical of reinforced concrete in early stages of cracking—then it is unlikely that the crack width could close in a far negative direction due to the compressive strength of concrete. However, concrete is weak in tension and it is much more probable that the crack width will widen over time.
Moreover, current methods of data collection are manually intensive, and require transcribing and recording measurements into engineering notebooks, and subsequent transfer to a computer for client reports. Current methods lack consistent mechanisms for engineering traceability, which adds further uncertainty to collected data. As many forensic studies are conducted for scientific evidence in litigation relating to failing foundations and structures, engineering traceability is a key component of a complete and accurate case.